Manufacture of near-net shape titanium alloy articles from metal powders by sintering at variable pressure

ABSTRACT

The process includes (a) mixing a titanium hydride powder having a particle size of ≦150 μm with alloying metal powders (master alloys or elemental metal powders) having a particle size in the range of {fraction (1/15)}-⅖ of the maximal particle size of titanium hydride powder, (b) compacting the resulting powder mixture by molding at the pressures of 400-1000 MPa, (c) heating up to the sintering temperature of the predetermined alloy composition at variable pressures in a furnace chamber: initial heating to 400° C. in vacuum of less than 10 −2  Pa, then, heating to a temperature range of 400-900° C. with the pressures up to 10 4  Pa, which is controlled by hydrogen being emitted by the decomposition of titanium hydride contained in the compacted powdered alloy, and finally, heating to over 900° C. to the sintering temperature at the pressure continually decreasing to the starting vacuum level, and (d) sintering. Heating to the sintering temperature is performed at the rate of 10-15 grad/min. The new technology allows the purity and mechanical properties of sintered titanium alloys and the manufacture of near-net shape sintered titanium articles to be controlled by a cost-effective method.

FIELD OF INVENTION

[0001] The present invention relates to powder metallurgy of titaniumalloys, and can be used in aircraft, automotive, Naval applications, oilequipment, chemical apparatus, and other industries. More particularly,the invention is directed at the manufacture of near-net shape titaniumarticles from sintered elemental and alloyed powders.

BACKGROUND OF THE INVENTION

[0002] Titanium alloys are well known to exhibit lightweight, highresisdence to oxidation or corrosion, as well as the highest specificstrength (the strength-to-weight ratio) amid all metals exceptberyllium. Previously, articles of titanium alloys have been produced bymelting, forming and machining processes, or by powder metallurgytechniques. The first method is not cost effective but provides highlevels of all properties of titanium alloys. The second method is costeffective but cannot completely realize all advantages of titaniumalloys.

[0003] Various processes have been developed during the last threedecades for the fabrication of near-net shape titanium articles withdesirable density and mechanical properties. The use of elemental powdermixtures, controlling the particle size distribution, vacuum sintering,hot isostatic pressing, and special surface finishing are among thosenew developments. But all of these processes, as well as conventionalpowder metallurgy techniques, impose certain limitations with respect tothe characteristics of the produced titanium alloys.

[0004] For example, a method for producing sintered articles from atitanium powder alloy disclosed in JP 06092605, 1998 includes molding amixture of elemental powders, vacuum sintering, hot isostatic pressingof the alloy in α+β region, and shot pinning to heal surface porosity.The irregular porosity in the interior portion of the sintered articlesis the drawback of this method, which decreases mechanical properties,especially the strength.

[0005] The method for producing titanium alloys from elemental powdersdisclosed in JP 129864, 1990, includes pressing of the powder mixture,vacuum sintering, quenching of the alloy in β-region, and hot pressingat a temperature over 800° C. The oxidation of resulting articles duringthe hot pressing results in the loss of mechanical properties.

[0006] The method described in the U.S. Pat. No. 4,432,795 includesgrinding particles of light metals to the particle size less than 20 μm,mixing them faith particles of titanium based alloys having a particlesize larger than 40 μm, and compacting the mixture by molding andsintering at temperatures less than that of a formation of any liquidphase. This method allows the manufacture of the alloy having a densityclose to the theoretical value but the resulting alloy, contaminated byoxygen, iron, and other impurities, also exhibits low mechanicalproperties.

[0007] The U.S. Pat. No. 4,838,935 describes the use of titanium hydridetogether with titanium powder in the primary mixture before molding andsintering. The molded article is heated in a hot-press vacuum chamber toa temperature sufficient for the dehydration of TiH₂ to remove to gases.Then, the article is heated to a temperature of 1350-1500° C. whilemaintaining the pressure and vacuum. This method cannot completelyprevent the oxidation of highly-reactive titanium powders during thesecond heating, because hydrogen is permanently outgassing from theworking chamber. Besides, this method is not suitable for powderedmixtures containing low-melting metal and phases.

[0008] A preliminary partial sintering of titanium and titanium hydridepowders with elemental powders of alloying metals is disclosed in U.S.Pat. No. 3,950,166. The “mother” alloy obtained in such a way ispulverized and remixed with powder metals such as Mo, V, Zr, and Al—Vmaster alloy to achieve the final composition of titanium alloy. Thismixture is molded in a predetermined shape and sintered at 1000-1500° C.in a vacuum. The preliminary sintering partially resolves one technicalproblem: how to improve uniform distribution of alloying components, butgenerates another: oxidation of the “mother” powder duringpulverization. Several attempts have been made to improve the densityand purity of sintered titanium alloys by using titanium hydride as theraw component, together with other alloying powders as in JP 07278609,1995, or JP 06088153, 1994, or U.S. Pat. No. 3,472,705, 1969, or WO9701409, 1997. All of these methods include vacuum heating and sinteringaccompanied with permanent outgassing. So, the “cleaning effect” ofhydrogen is not used properly, and partial oxidation reoccurs after theremoval of hydrogen from the vacuum chamber. Thus, these methods do notprovide an effective improvement of mechanical properties of sinteredalloys, in spite of the sintering promoted by thermal dissociation oftitanium hydride.

[0009] Some specialized technologies were offered to manufacturetitanium alloys in hydrogen atmosphere in JP 58034102, 1983 and CH684978, 1995. These methods cannot prevent the contamination of sinteredmetals as well as the methods mentioned above: after the replacement ofa hydrogen-containing atmosphere by an inert gas, the oxidation ofreactive powders reoccurs.

[0010] All other known processes for making near-net shape titaniumalloys from metal powders have the same drawbacks: (a) insufficientpurity and low mechanical properties of sintered titanium alloys, (b)irregular porosity and insufficient density of sintered titanium alloys,and (c) low reproduction of mechanical properties that depend on thepurity of raw materials.

OBJECTIVES OF THE INVENTION

[0011] The object of the invention is to increase the mechanicalproperties, particularly strength and plasticity, of near-net shapearticles manufactured by sintering titanium alloys from elemental and/oralloyed metal powders.

[0012] In order to obtain a high level of mechanical properties, anyoxidation or contamination of powdered components must be preventedduring heating and sintering.

[0013] Another objective of the present invention is to provide lowporosity and high-density structures of sintered titanium alloys toachieve the densities close to the theoretical value.

[0014] It is also an objective to provide the cost-effective manufactureof near-net shape articles using one run heating and sintering ofpowdered titanium alloys.

[0015] The nature, utility, and further features of this invention willbe more apparent from the following detailed description, with respectto preferred embodiments of the invented technology.

SUMMARY OF THE INVENTION

[0016] The invention relates to the manufacture of near-net shapetitanium articles from sintered powders containing titanium and allrequired alloying elements. While the manufacture of titanium alloys bysintering elemental and alloyed metal powders including titanium hydridehas previously been contemplated as mentioned above, problems related toinsufficient strength, irregular porosity, insufficient density, andcost reductions have not been solved.

[0017] The invention overcomes these problems by:

[0018] (a) mixing a titanium hydride powder having a particle size of≦150 μm with alloying metal powders (master alloys or elemental powders)having a particle size in the range of {fraction (1/15)}-⅖ of themaximal particle size of said titanium hydride powder,

[0019] (b) compacting the resulting powder mixture by molding at thepressure of 400-1000 MPa,

[0020] (c) heating to the sintering temperature of the predeterminedalloy composition at variable pressure in the furnace chamber: initiallyheating to 400° C. in vacuum of less than 10⁻² Pa, then, increasing thetemperature to a range of 400-900° C. with the pressure up to 10⁴ Pa,which is controlled by hydrogen being emitted by the decomposition oftitanium hydride contained in the compacted powdered alloy, and finally,heating to over 900° C. to the sintering temperature with the pressurecontinually decreasing to the starting vacuum level, and,

[0021] (d) sintering.

[0022] Heating to the sintering temperature is performed at the rate of10-15 grad/min.

[0023] In another aspect of the invention, technology is provided tomanufacture near-net shape sintered titanium articles in acost-effective way.

[0024] In essence, the core of the invention is to control the purityand mechanical properties of sintered titanium alloys using (a) TiH₂powder having a predetermined particle size as the base component, (b)optimal ratio of particle size between TiH₂ powder and alloyed metalpowders, and (c) variable pressure of hydrogen in the furnace chamberduring the heating and sintering.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS OF THE INVENTION

[0025] As discussed, the present invention relates generally to themanufacture of sintered titanium alloys using elemental metal powdersand titanium hydride as raw materials. Optimal size distribution of rawmetal powders and the prevention of their oxidation during heating andsintering play a very important role in such processes.

[0026] No previously known methods, also mentioned in References, triedto find out the optimal ratio between the particle size of titaniumhydride powder and the particle size of metal powders alloying thetitanium base. The known methods always used permanent outgassing of thevacuum chamber during heating and sintering. Therefore, a completereaction is not achieved between metal powders and green titaniumcompacts with hydrogen, and the final structure of the sintered alloycontains oxides and irregular porosity.

[0027] On the one hand, the particle size of the titanium base andalloying metal powders should be as small as possible to enhance thechemical homogenization and to reach high final density of the sinteredalloy. On the other hand, the smaller the particle size of raw metalpowders, the more chemical contaminants in the powder mixture to bemolded and sintered. We found that a combination of titanium hydridepowder having a particle size of <150 μm with alloying metal powdershaving a particle size in the range of {fraction (1/15)}-⅖ of the sizeof said titanium hydride powder is the optimum to obtain a fully dense,strong structure of resulting titanium alloy. These sizes of raw metalpowders achieve a high rate of homogenization and higher density of thesintered alloy accompanied with the limitation of impurities at thedesired lower level.

[0028] Experimental testing of titanium hydride powder having a particlesize >150 μm showed that the final density of the sintered alloy wasdecreased to 98% of the theoretical value and less. The use of otherpowdered components with a particle size less than {fraction (1/15)} ofthe particle size of the TiH₂ powder resulted in unacceptablecontamination of the sintered alloy.

[0029] The use of titanium hydride powder as the base component insteadof titanium powder, promotes rapid phase formation and activation ofsintering of powdered preforms. The titanium hydride is decomposedduring the vacuum heating with the emission of hydrogen in the range of400-900° C. that results in the formation of titanium having highdensity of crystalline defects, and hastens the acceleration of thediffusion process.

[0030] The emitted atomic hydrogen beneficially effects on sinteringkinetics, reduces any oxides that are usually located on the surface ofpowder particles, and by doing so, is cleaning interparticle interfacesand enhances the diffusion between all components of the powder mixture.

[0031] In order to use this positive effect, it is necessary to maintaina high concentration of hydrogen in molded preforms and provide itspermanent emission during the heating process to the sinteringtemperature. We increase the partial pressure of hydrogen in the furnacechamber up to 10⁴ Pa in the temperature range of 400-900° C. to keephydrogen in the crystalline lattice of titanium up to 900° C. Highpressure of ambient hydrogen prevents a decrease in the concentration ofhydrogen dissolved in titanium that usually happens with an increase intemperature. Further heating and sintering is carried out by outgassinghydrogen from the working chamber to the remaining pressure of 10⁻² Pato remove the hydrogen from the metal and to transform the multiphasepowder mixture into a chemically homogeneous and fully dense alloy. Suchchange in the hydrogen pressure during the processing of titaniumpowders increases mechanical properties of the resulting alloy,especially the strength and plasticity. Thus, the positive effect ofhydrogen is used in the heating stage. The hydrogen cannot be present inthe vacuum chamber during the final stages of sintering in order toprevent its negative effect on properties of the solid sintered alloy.

[0032] On the other side, the particle size of alloying powders in thealloy mixture should not be larger than ⅖ of the particle size of basetitanium powder to provide a complete solid-phase reaction oflow-melting powders (for example, elemental aluminum) with titanium,before they reach their melting points to avoid a significantKirkendal-type porosity. The use of larger low-melting powders resultedin the partial or even complete liquid-phase reaction with the titaniumbase, because coarse low-melting powders or their eutectics are meltedduring the heating earlier than when the solid-phase reaction wouldoccur. This premature liquid-phase reaction on the heating stageresulted in an incomplete homogenization of the alloy composition thatcannot be improved by subsequent sintering and annealing.

[0033] Thus, the above-mentioned ratio of particle sizes betweentitanium hydride powder and other metal powders in the raw mixture wasexperimentally proven, and can be considered as the optimal ratio.

[0034] Molding of powdered preforms to desired near-net shape is carriedout at the pressure of 400-1000 MPa. The pressure less than 400 MPa isinsufficient for molding. On the other hand cracks in the moldedpreforms occur at the pressure of >1000 MPa. The heating of moldednear-net shape preforms is carried out at a rate of 10-15° C./min toprovide the technological quality of the process. The preforms crack atthe rate of >15° C. min because the hydrogen emission from thedecomposed titanium hydride is too intense. The rate of <10° C./min istoo low and has no affect on any properties of the processed alloy.

[0035] The hydrogen emission at a temperature less than 400° C. isinsignificant, and the temperature higher than 900° C. is nearlycomplete. Therefore, the pressure of hydrogen is controlled in theworking chamber in the temperature range of 400-900° C.

[0036] The return of the pressure in the working chamber to the level ofless than 10⁻² Pa accompanied with the heating from 900° C. to thesintering temperature results in the elimination of hydrogen from thesintered alloy. The absence of hydrogen prevents the deterioration ofmechanical properties of titanium alloy, especially preventing ahydrogen-ignited brittleness.

[0037] The innovated technology allows the manufacture of chemicallyhomogeneous titanium alloys with high densities and mechanicalproperties compared to properties of casting alloys.

EXAMPLE 1

[0038] Titanium hydride powder having a particle size of <150 μm wasmixed with master alloys Ti—Al and Al—V powders having a particle sizeof −10 . . . −60 μm in the ratio providing the stoicheometriccomposition of the alloy Ti-6Al-4V. Powders are mixed for 6 hours, andcompacted (molded) at 700 MPa in the near-net shape preform having arelative density of 74%. The preform was heated in a vacuum of 10⁻² Paat the rate of 10° C./min up to 1350° C. No liquid phases were at thistemperature, yet. During the heating process, the pressure in thefurnace chamber was increased to 10⁴ Pa in the temperature range of400-900° C. resulting in hydrogen being emitted from the titaniumhydride. The pressure in the chamber was decreased gradually to 10⁻² Paduring heating to over 900° C. Then, the preform was sintered for 4hours at 1350° C. The obtained article was studied using microstructuralanalysis, X-ray, and microspectral analysis, which confirmed that theproduced metal is a chemically and structurally homogeneous alloyTi-6Al-4V having a density of 98.9% of the theoretical value. Thetensile strength of the obtained alloy was 960 MPa and the elongationwas 7%.

EXAMPLE 2

[0039] Titanium hydride powder having a particle size of <100 μm wasmixed with aluminum and vanadium powders having a particle size of +10 .. . −30 μm in the ratio providing the stoicheometric composition of thealloy Ti-6Al-4V. Powders are mixed for 5 hours, and compacted (molded)at 800 MPa in the near-net shape preform having a relative density of76%. The preform was heated in a vacuum of 10⁻² Pa at the rate of 10°C./min up to 1250° C. During the heating process, the pressure in thefurnace chamber was increased to 10⁴ Pa in the temperature range of400-900° C. resulting in hydrogen being emitted from the titaniumhydride. Aluminum 45 powder reacts with titanium base at 600-620° C.,which is lower than the melting temperature of aluminum. The pressure inthe chamber was decreased gradually to 10⁻² Pa during heating to over900° C. Then, the preform was sintered for 4 hours at 1250° C. Theobtained article was studied using microstructural analysis, X-ray, andmicrospectral analysis, which confirmed that the produced material is achemically and structurally homogeneous alloy Ti-6Al-4V having a densityof 98.7% of the theoretical value. The tensile strength of the obtainedalloy was 990 MPa and the elongation was 3%.

EXAMPLE 3

[0040] Titanium hydride powder having a particle size of <150 μm wasmixed with master alloys Mo—Al and Al—V powders having a particle sizeof +10 . . . −60 μm in the ratio providing the stoicheometriccomposition of the alloy Ti-3Al-5Mo-5V. Powders are mixed for 6 hours,and compacted (molded) at 700 MPa in the near-net shape preform having arelative density of 75%. The preform was heated in a vacuum of 10⁻² Pawith the rate of 15° C./min up to 1300° C. No so liquid phases were atthis temperature, yet. During the heating process, the pressure in thefurnace chamber was increased to 10⁴ Pa in the temperature range of400-900° C. resulting in hydrogen being emitted from the titaniumhydride. The pressure in the chamber was decreased gradually to 10⁻² Paduring heating to over 900° C. Then, the preform was sintered for 7hours at 1300° C. The obtained article was studied using microstructuralanalysis, X-ray, and microspectral analysis, which confirmed that theproduced metal is a chemically and structurally homogeneous alloyTi-3Al-5Mo-5V having a density of 98.4% of the theoretical value. Thetensile strength of the obtained alloy was 920 MPa and the elongationwas 5%.

EXAMPLE 4

[0041] Titanium hydride powder having a particle size of <150 μm wasmixed with aluminum and molybdenum powders having a particle size of +10. . . −30 μm in the ratio providing the stoicheometric composition ofthe alloy Ti-6Al-3Mo. Powders are mixed for 7 hours, and compacted(molded) at 600 MPa in the near-net shape preform having a relativedensity of 73%. The preform was heated in a vacuum of 10⁻² Pa at therate of 10° C./min up to 1250° C. During the heating process, thepressure in the furnace chamber was increased to 1 Pa in the temperaturerange of 400-900° C. resulting in hydrogen being emitted from titaniumhydride. Aluminum powder reacts with titanium base at 620-640° C., whichis lower than the melting temperature of aluminum. The pressure in thechamber was decreased gradually to 10⁻² Pa during heating to over 900°C. Then the preform was sintered for 7 hours at 1250° C. The obtainedarticle was studied using microstructural analysis, X-ray, andmicrospectral analysis, which confirmed that the produced metal is achemically and structurally homogeneous alloy Ti-6Al-3Mo having adensity of 98.2% of the theoretical value.

[0042] The innovated technology is suitable for applications both in alab testing and a serial manufacture of sintered articles from titaniumalloys.

We claim:
 1. The manufacture of near-net shape titanium alloy articlesincludes: (e) mixing a titanium hydride powder having a particle size of<150 μm with alloying metal powders (master alloys and/or elementalpowders) having particle sizes in the range of {fraction (1/15)}-⅖ ofthe maximal particle size of said titanium hydride powder, (f)compacting the obtained powder mixture by molding at the pressure of400-1000 MPa, (g) heating to the sintering temperature of thepredetermined alloy composition at variable pressures in the furnacechamber: initially heating to 400° C. in vacuum of less than 10⁻² Pa,then, heating in a range of 400-900° C. at pressure up to 10⁴ Pacontrolled by hydrogen being emitted due to the decomposition oftitanium hydride contained in the compacted powdered alloy, and finally,heating to over 900° C. to the sintering temperature at the pressurecontinually decreasing to the starting vacuum level, and, (h) sintering.2. The manufacture of near-shape titanium alloy articles according toclaim 1, wherein the heating to the sintering temperature is performedwith the rate of 10-15 grad/min.